The Data Binding Library is a support library that allows you to bind UI
components in your layouts to data sources in your app using a declarative
format rather than programmatically.

Layouts are often defined in activities with code that calls UI framework
methods. For example, the code below calls findViewById() to find a TextView widget and bind it to the userName property of the
viewModel variable:

Kotlin

Java

The following example shows how to use the Data Binding Library to assign text
to the widget directly in the layout file. This removes the need to call any of
the Java code shown above. Note the use of @{} syntax in the assignment
expression:

<TextView
android:text="@{viewmodel.userName}" />

Binding components in the layout file lets you remove many UI framework calls in
your activities, making them simpler and easier to maintain. This can also
improve your app's performance and help prevent memory leaks and null pointer
exceptions.

Using the Data Binding Library

Use the following pages to learn how to use the Data Binding Library in your
Android apps.

The expression language allows you to write expressions that connect
variables to the views in the layout. The Data Binding Library automatically
generates the classes required to bind the views in the layout with your data
objects. The library provides features such as imports, variables, and
includes that you can use in your layouts.

These features of the library coexist seamlessly with your existing layouts.
For example, the binding variables that can be used in expressions are defined
inside a data element that is a sibling of the UI layout's root element.
Both elements are wrapped in a layout tag, as shown in the following
example:

The Data Binding Library provides classes and methods to easily observe data
for changes. You don't have to worry about refreshing the UI when the
underlying data source changes. You can make your variables or their
properties observable. The library allows you to make objects, fields, or
collections observable.

For every layout expression, there is a binding adapter that makes the
framework calls required to set the corresponding properties or listeners. For
example, the binding adapter can take care of calling the setText() method
to set the text property or call the setOnClickListener() method to add a
listener to the click event. The most common binding adapters, such as the
adapters for the android:text property used in the examples in this page,
are available for you to use in the android.databinding.adapters package.
For a list of the common binding adapters, see
adapters.
You can also create custom adapters, as shown in the following example:

Java

The Android Support Library includes the Architecture
Components, which you can use to
design robust, testable, and maintainable apps. You can use the Architecture
Components with the Data Binding Library to further simplify the development
of your UI.

The Data Binding Library supports two-way data binding. The notation used for
this type of binding supports the ability to receive data changes to a property
and listen to user updates to that property at the same time.